Skip to buttons on tabbing in issues page

This will skip tabbing right to "Add comment" or "close" buttons.

http://www.w3.org/TR/html5/editing.html#sequential-focus-navigation-and-the-tabindex-attribute

For tabindex:

```
If the value is a negative integer
The user agent must set the element's tabindex focus flag, but should not allow the element to be reached using sequential focus navigation.
```